Since those are removable devices being external the first place to go would be the Disk Management tool in order to assign each a new drive letter. Apparently they simply haven't been initialized with Windows already having assigned one to each when mounting the volume on each as a logical drive.
The item when seen in the DM will likely be seen as "offline" where you would first right click on the square to the left of the main partition area displayed there in order to click on the "online" option in the right click menu. Once done you right click a second time on the main volume to select the change drive letter option unless you are presented with the add new volume screen.
The first image attached shows a new drive without any partition seen as "raw" and being set to online prior to the new partition to be created. The second is from the add new volume wizard if needed if the drives are found offline and then see the wizard appear once set to online.